Dehydrocyclization is a well known reaction wherein alkanes are converted to aromatics. For example, hexane may be dehydrocyclized to benzene, heptane to toluene, etc.
Catalytic reforming is a well-known refinery process for upgrading light hydrocarbon feedstocks, frequently referred to as naphtha feedstocks. Products from catalytic reforming can include high octane gasoline, useful as automobile fuel, and/or aromatics, such as benzene and toluene, useful as chemicals. Reactions typically involved in catalytic reforming include dehydrocyclization, isomerization and dehydrogenation. The dehydrocyclization and dehydrogenation reactions lead to the production of aromatics starting, respectively, from linear and slightly branched alkanes of a proper size, e.g., 6-8 carbon atoms and from cycloalkanes, e.g., cyclohexane and methyl-, dimethyl- and ethyl-cyclohexanes. Because typical petroleum feedstocks are much richer in linear and slightly branched alkane than in cycloalkanes, dehydrocyclization tends to be the more important of these reactions. Thus, reforming typically includes dehydrocyclization. However, dehydrocyclization or aromatization of alkanes can be directed more narrowly than reforming.
For a long period of time, the leading catalyst used in reforming was platinum on an alumina-halide support. This catalyst had some sensitivity to sulfur, but modest amounts of sulfur, such as 10 to 100 ppm or more were acceptable and sometimes preferred.
In the late 1960's and early 1970's, a catalyst was introduced which had more sensitivity to sulfur. This catalyst contained platinum and rhenium, generally on an alumina-halide support. This catalyst was found to perform best at sulfur levels in the feed below 10 ppm, preferably below 1 ppm; see U.S. Pat. No. 3,415,737 to Kluksdahl. The catalyst preferably was sulfided prior to use in catalytic reforming.
More recently, U.S. Pat. No. 4,634,518 to Buss and Hughes disclosed a process for catalytic reforming and/or dehydrocyclization/aromatization wherein the catalyst is even more sensitive to sulfur than the platinum rhenium catalyst. The U.S. Pat. No. 4,634,518 process uses a catalyst such as platinum on a large pore crystalline aluminosilicate zeolite, such as L-zeolite. Preferably, the sulfur is maintained at less than 0.1 ppm in the feed to the aromatization process. See U.S. Pat. No. 4,456,527 to Buss, Field and Robinson.
U.S. Pat. No. 4,835,336 to McCullen discloses sulfiding a noble metal/low acidity medium pore size zeolite catalyst to suppress hydrogenolysis and increase aromatic selectivity of the catalyst. The silica to alumina ratio according to the '336 patent is at least 12. The example in the '336 patent discloses a silica to alumina ratio of 26,000. The amount of alkali in the '336 catalyst is not disclosed in the Example in that patent. With regard to inclusion of alkali in the '336 catalyst, the '336 patent teaches at column 6, line 9:
"the low acidity zeolite (for example, ZSM-5) can be synthesized to have a low aluminum content, or may be exchanged with Group IA or IIA cations to reduce acidity." PA1 "the zeolites used as catalysts in this invention may be in the hydrogen form or they may be base exchanged or impregnated to contain ammonium or a metal cation complement. The metal cations that may be present include any of the cations of the metals of Group I through VIII of the periodic table. However, in the case of Group IA metals, the cation content should in no case be so large as to substantially eliminate the activity of the zeolite for the catalysts being employed in the instant invention."
At column 12, line 1, the '336 patent teaches:
Japanese Kokai 115087, laid open May 26, 1987, discloses the use of a high silica to alumina ratio zeolite for reforming. The catalyst contains a Group IIB metal and is presulfided.
Another reference which discloses the use of zeolitic catalyst in aromatization or dehydrocyclization reaction is U.S. Pat. No. 4,347,394, to Detz and Field. The catalyst disclosed for use in the process of the '394 Detz and Field patent contains a crystalline aluminosilicate which is commonly referred to as silicalite. Silicalite is generally regarded as having the same basic X-ray diffraction pattern as the well-known zeolite ZSM-5. ZSM-5 is disclosed in U.S. Pat. No. 3,702,886 to Argauer. However, the ratio of silica to alumina for silicalite is higher than is the silica/alumina ratio for typical ZSM-5 catalyst.
According to the Detz and Field patent cited above, the sulfur in the feed is low, less than 0.2 ppm for feed 1, as disclosed in Column 5 of that patent, and less than 0.02 ppm for feed 2. Also, the acidity of the catalyst is low, so that the catalyst is referred to as a non-acidic catalyst. According to the Column 6 examples in the Detz and Field patent, the amount of alkali used in the catalyst is not necessarily low. Thus, Column 6, Experiments A, B and C, show that 0.017 wt. % sodium was too low a sodium value to produce good yields of C.sub.5 + and of benzene, whereas, at higher sodium values better yields resulted. At 0.99 wt. % sodium in the catalyst, the C.sub.5 + yield and benzene yield improved substantially and at 4.12 wt. % sodium, the improvement was even greater.
As will be seen from the description below, the present invention requires the use of catalysts that are sulfur tolerant.
U.S. Pat. No. 4,680,280 to Pandey and Buss discloses the addition of molybdenum to zeolite L-catalytic reforming catalyst as a means of improving sulfur tolerance. U.S. Pat. No. 4,579,831 to field discloses a sulfur resistant catalyst comprising a zeolite bound with alumina containing an alkali or alkaline earth component.
U.S. Pat. No. 4,401,555 to Miller is directed to olefin production from paraffins using silicalite having a low sodium content. The silicalite used in the '555 process contains less than 0.1 wt. % sodium and is composited in a matrix which is substantially free of cracking activity. Also, the composite has no hydrogenation component. According to the '555 process, the feed can be hydrotreated to reduce sulfur to less than 100 ppm organic sulfur.
U.S. Pat. No. 4,851,605 to Bortiger, et al discloses a method of making a zeolite, such as ZSM-5, on a pH controlled sodium free basis. The catalyst of U.S. Pat. No. 4,851,605 is used in a process to synthesize olefins from methanol and/or dimethyl ether.